1. Field of the Invention
The present invention relates to a substrate of an active matrix type liquid crystal display device which displays an image by applying driving signals to pixel electrodes via switching elements for display so as to generate a voltage difference between the pixel electrodes and the electrodes opposed thereto.
2. Description of the Related Art
In a conventional active matrix type liquid crystal display device, a plurality of independent pixel electrodes are disposed in a matrix, and a switching element is provided in each of the pixel electrodes. Switching elements connected to a scanning line are selected by a scanning signal provided to the switching elements, and a signal voltage is applied from signal lines to each pixel electrode through the switching element upon application of the scanning signal to the switching elements. A liquid crystal layer interposed between the pixel electrodes and a counter electrode forms a plurality of pixels and each of the pixels is subjected to optical modulation due to the electric potential difference between the pixel electrode and the counter electrode. As a result, the modulation of the plurality of the pixels produces a display pattern. As a switching element for selectively driving the pixels, a thin film transistor (TFT), a MIM (Metal-Insulator-Metal) element and the like are generally used.
In the case where such a display device is used for displaying an image, the signal voltage applied to the pixel electrodes at a certain scanning timing is required to be sufficiently held in the pixel electrodes until the next scanning timing. For this reason, while the switching elements are in an off-state, the pixel electrode and the signal line are required to be electrically isolated by high resistance. In other words, a liquid crystal capacitor consisting of the pixel electrode, the counter electrode and the liquid crystal layer therebetween, and the signal line is required to be electrically isolated by high resistance.
In the case of a transmission type liquid crystal display device, it is necessary for the portions where the pixel electrodes are located to be transparent. Accordingly, a transparent insulating film is used as an insulating film between scanning lines and signal lines, and the pixel electrodes and the signal lines are isolated by the transparent insulating film. As a result, in order to shield light which leaks from the lateral separation between the pixel electrodes and the signal lines, a light-shielding film having light-transmitting portions only in the areas corresponding to the pixel electrodes is provided generally on the counter substrate facing to the active matrix substrate. In view of an error in alignment of the active matrix substrate with the counter substrate occurring during the manufacturing process, the width of the light-shielding portions of the shielding film on the side of the counter substrate is made larger than that of the pixel electrodes. In other words, the area of the light-transmitting portions is smaller than that of the pixel electrodes. This is one of the factors which prevents an aperture ratio of the pixels from improving in the active matrix type display device.
A first conventional example of an active matrix substrate is constructed as shown in FIGS. 5 and 6. In FIGS. 5 and 6, gate bus lines 12, which are scanning lines, are provided on a glass substrate 11 having an insulation property. A transparent insulating film 13 made of a silicon type compound such as silicon nitride (SiN.sub.x), silicon oxide (SiO.sub.2) and the like is formed on the gate bus lines 12 and the glass substrate 11. Source bus lines 14, which are signal lines, are provided on the transparent insulating film 13 so as to cross the gate bus lines 12. Pixel electrodes 15 are formed in areas surrounded by both the bus lines 12 and 14. There are lateral separations E between the pixel electrodes 15 and the source bus lines 14 (FIG. 6).
Video signals (signal voltages) supplied from the source bus lines 14 are applied to the pixel electrodes 15 via thin film transistors 16 acting as switching elements. A light-shielding film 17 is provided over the gate bus lines 12, the source bus lines 14 and the lateral separations E, but excluding all but a small portion of the pixel electrodes 15, the small portion being the edge facing toward the source bus line 14.
In the active matrix substrate constructed in such a manner as the first conventional example, light irradiated by a light source (not shown in Figures) from the side of the glass substrate 11 is shielded by the light-shielding film 17 so as not to leak onto the display portion other than the pixel electrodes 15.
In the conventional active matrix substrate, it is intended to prevent light from the light source from the side of the glass substrate 11 from leaking to the display portions other than the portions corresponding to the pixel electrodes 15. However, it is not taken into consideration to prevent light from the light source from the side of glass substrate 11 from being irradiated to the transparent insulating films 13 located in the lateral separations E between the pixel electrode 15 and the source bus lines 14.
Generally speaking, when a semiconductor layer is irradiated with light, a leakage current of the switching element in an off-state is increased due to photo-excitation. As a result, the voltage of the pixel electrodes 15 is not sufficiently maintained, resulting in deterioration in display quality. Therefore, the light-shielding film is provided so as to prevent the switching element from being irradiated with light from the light source for the purpose of preventing poor display quality.
A second conventional example of active matrix substrates is constructed as shown in FIGS. 7 and 8. In FIGS. 7 and 8, gate bus lines 22, which are scanning lines, are provided on a glass substrate 21 having an insulation property. A transparent insulating film 23 made of a silicon type compound such as silicon nitride (SiN.sub.x), silicon oxide (SiO.sub.2) and the like is formed on the gate bus lines 22 and the glass substrate 21. Source bus lines 24, which are signal lines, are provided on the transparent insulating film 23 so as to cross the gate bus lines 22. Pixel electrodes 25 are formed in areas surrounded by both the bus lines 22 and 24 with predetermined lateral separations between the bus lines 22 and 24 and the pixel electrodes 25.
Video signals delivered from the source bus lines 24 are supplied to the pixel electrodes 25 via thin film transistors 26 acting as switching elements.
The thin film transistor 26 is constructed as shown in FIG. 8. A semiconductor layer 27 is formed between the glass substrate 21 and the transparent insulating film 23. A source contact 28 and a drain contact 29 are formed in portions of the semiconductor layer 27 and a source electrode 30 and a drain electrode 31 are formed on the respective contacts 28 and 29 through openings formed in a predetermined portion of the transparent insulating film 23. A gate electrode 33 is formed on a gate insulating film 32, which is the transparent insulating film located between the source electrode 30 and the drain electrode 31, with a predetermined lateral separation therebetween.
A light-shielding film having a light-shielding portion 34 which is slightly larger than the semiconductor layer 27 is formed between the glass substrate 21 and the semiconductor layer 27.
In the active matrix substrate constructed in such a manner as the second conventional example, light irradiated by a light source (not shown in Figures) from the side of the glass substrate 21 to the semiconductor layer 27 is shielded by the light-shielding film 34.
However, the inventors of the present invention have noticed the following problems. In the active matrix substrate constructed in such a manner as described above, silicon type compound such as silicon nitride, silicon oxide and the like is used as the material for the transparent insulating films 13 and 23. In the case of such transparent insulating films 13 and 23, the microstructure (crystal structure) of the transparent insulating films 13 and 23 are changed, or an abnormal energy level generated on the surfaces of transparent insulating films 13 and 23, due to the difference in film-forming conditions and contamination by different elements. As a result, the insulating property of the transparent insulating film is deteriorated. In the case where such defects exist in the transparent insulating films 13 and 23, particularly, when the portions having the defects are irradiated with light, electrons in the transparent insulating films 13 and 23 are excited by the light irradiation (photo-excitation). As a result, the insulating property of the transparent insulating films deteriorates in accordance with the intensity of the light. Thus preventing charges from being sufficiently retained at the pixel electrodes 15 and 25. Accordingly, an image of high quality cannot be obtained.
It is known that when the transparent insulating films 13 and 23 are continuously irradiated with intense light, the microstructure of the transparent insulating films 13 and 23 deteriorates and the inherent properties thereof are deteriorated. In the case of the transparent insulating films 13 and 23, this causes the deterioration in the insulating property, leading to the deterioration in display quality such as the deterioration in display contrast and the like. In particular, when used in a liquid crystal projection display apparatus or the like, a liquid crystal panel including the active matrix substrate is exposed to high intensity light. Thus, the deterioration of the transparent insulating films 13 and 23 due to such light causes a severe problem in terms of durability and reliability.